The radiation dose response characteristics of the spinal cord will be investigated for dose fractionation regimens using fractional doses between 1 and 4 Gy. These doses are relevant not only to standard radiotherapy but also to increasingly-used hyperfractionated and accelerated regimens which employ multiple smaller-than-standard doses per day. Myelopathy is the worst complication of radiotherapy but the fractionated dose response in the 1-4 Gy range is almost completely undocumented. Available data are inadequate for clinical application. The interval between dose fractions is also important since some data suggest that repair of sublethal damage is incomplete at 6 hours and may continue for longer than 12 hours. If significant repair occurs after 6-8